Landmines and mine-like improvised explosive devices (“IEDs”) are weapons that are typically buried under the soil surface or other debris in order to avoid detection. The devices generally include an initiator, a detonator, and a bulk explosive charge. The initiator may be a built-in trigger such as a pressure plate, a magnetic sensor, or an acoustic sensor, or, particularly in the case of IEDs, a receiver for a remote triggering device such as a detonator box, a phototrigger, or a cellular phone. The detonator is generally one of a plethora of small primary explosive devices, such as a blasting cap, that can be embedded within or placed in direct contact with the bulk explosive charge. The combined explosives are generally enclosed within a housing to protect them prior to detonation. This housing could be variously constructed from metal (for convenience, to shape the force of the explosive charge, or to serve as a source of fragmentation projectiles), plastic (to avoid detection by metal detection technologies), or, particularly in the case of IEDs, convenience materials such as glass or wood. The least variable element in these devices tends to be the bulk explosive charge, which is generally a secondary explosive, such as TNT (trinitrotoluene), RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), or PETN (pentaerythritol tetranitrate), but in the case of IEDs may sometimes be a less stable primary explosive, such as TATP (triacetone triperoxide) or TNP (trinitrophenol, also known as “picric acid”). While explosives obtained from some commercial sources may contain a semi-volatile chemical taggant, such as EGDN (ethylene glycol dinitrate), DMDNB (2,3-dimethyl-2,3-dinitrobutane), or mononitrotoluenes (o-MNT and/or p-MNT), military-grade and “homebrewed” explosives will lack such taggants, making the detection of an explosive compound itself the most reliable method for detecting a buried landmine or IED.
In landmines and mine-like IEDs, the devices are usually buried within about 0.5 meters of the ground surface, with the typical depth varying from centimeters for small anti-personnel devices to tens of centimeters for larger anti-vehicular devices. Thus, both ground-penetrating and surface sensing technologies have been employed to detect these devices. For example, the bulk explosive charge, as well as any initiator, detonator, and housing, will generally have a different acoustic impedance from the soil or debris in which it is buried, reflecting at least a portion of the energy emitted by an acoustic detection system away from the buried device. As shown in FIG. 1, where solid lines represent empirical measurements and dashed lines represent extrapolations of the empirical measurements across the illustrated temperature range, most explosive materials will also have a slight but non-negligible vapor pressure in comparison to normal atmospheric pressure (760 Ton or 109 ppbv, located at approximately the top of the graph), so that trace amounts of the explosive charge will tend to disperse into the soil and air above a buried device.
Acoustic technologies have previously been applied to the problem of landmine detection, but in the main only through an analysis of the acoustic energy reflected as a result of the aforementioned difference in acoustic impedance. In a first technique, a loudspeaker emits an acoustic pulse over a suspected device, a microphone array measures the reflected acoustic energy, and the microphone data is rapidly analyzed to determine whether the array has detected a significant deviation from background acoustic reflections, indicating a buried object. See, for example, John A. Waschl's discussion of differential acoustic reflection technologies in A Review of Landmine Detection Technologies, Report No. DSTO-TR-0113, published by the Aeronautical and Maritime Research Laboratory of the Commonwealth of Australia (1994). In a second technique, a loudspeaker (or, potentially, a laser) generates an acoustic pulse near a suspected device, a laser Doppler device measures the motion of the ground surface caused by the acoustic pulse and any reflected acoustic energy, and the laser Doppler data is rapidly analyzed to detect anomalous surface motion caused by a change in acoustic impedance or acoustic reflection, indicating a buried object. See, for example, J. C. van den Heuvel et al.'s disclosure of a laser Doppler vibrometer system in Laser-induced Acoustic Landmine Detection with Experimental Results on Buried Landmines, Proceedings of SPIE, Vol. 5415, pp. 51-60 (2004). The second technique is a significant advance over the first, since the laser Doppler device can comparatively rapidly scan an area from a distance, permitting the standoff detection of shallowly buried objects within an intended path of travel. However, while these technologies can detect buried landmines and IEDs, they are also susceptible to “false targets”—i.e., they tend to detect all buried objects that appear acoustically similar to a landmine or other target device, regardless of whether that object is an explosive device, a decoy device, or merely buried litter.
Light-based standoff chemical sensing technologies, such as spectroscopy, have also been applied to the problem of landmine detection, but are poorly suited for detecting buried, low-volatility explosives since they typically seek to detect trace chemical vapors in the most dynamic part of the environment near a buried device—the air above the ground surface. See, for example, Suman Singh's discussion of laser-induced breakdown spectroscopy (“LIBS”) in Sensors—An Effective Approach for the Detection of Explosives, Journal of Hazardous Materials, Vol. 144, pp. 15-28 (2007). Scientists and engineers seeking to improve standoff explosives detection technology have recently begun to combine light-based standoff chemical sensing technologies with heating techniques in order to increase the concentration of trace chemical vapors in the air and improve the effective sensitivity of their systems. An example of such a combination, disclosed in U.S. Pat. No. 7,796,264, uses a remote heat source such as a microwave beam to heat a target material in order to increase the vapor pressure of the target's constituent materials and consequently the concentrations of chemical vapors found in the air above the target. However, while such a system could be modified to sequentially heat the ground surface in order to search for an unseen and buried explosive device, such a modified system would need to heat large areas of the ground surface, and the speed of the modified system would be limited by the power that could be projected to heat an intended path of travel.
As a result, the applicants have perceived a need for an enhanced chemical sensing system that is better suited for use with buried, low-volatility chemicals, and in particular better suited for standoff use in the remote detection of buried explosive charges such as those found in landmines and IEDs. Such a system should be reasonably selective for the chemical species of interest, with a lower false positive rate than known acoustic detection technologies, but require less power than spectroscopic systems combined with wide-area heating techniques.